The field of the present invention is precise metal etching.
Thin gauge precision metal parts have been produced by photochemical fabrication processes. One such process consists of coating a clean metal sheet with a layer of photosensitive resistive material, then covering it with a photomask. The photomask defines covered areas and exposed areas to achieve a pattern of the final product. The material directly under the photomask remains solvent dissolvable after ultraviolet radiation is applied over the mask. The photoresist layer under the clear area of the photomask is hardened when the workpiece is exposed to ultraviolet radiation. The hardened photoresist covers the desired portion of the metal work piece and forms a chemically-resistant etch mask, leaving the unwanted portion to the attack of an etchant, such as a chromic acid solution or ferric chloride. After chemical etching, the hardened photoresist can be removed by a hot organic stripper. This method has its limitations. For example, it cannot produce a clearly defined, straight, sharp and narrow slit of 0.001 inches in a 0.01 inch thick metal sheet.
Another method, generally referred to as the bi-metal method is capable of greater accuracy. In this process, a temporary photoresist etch mask is replaced by a permanent etch mask such as nickel, gold or silver. In the aforementioned process, the photomask is a negative. In this process, the photomask is a positive of the product resulting from the final etch. The photoresist is covered by an appropriate mask and subjected to ultraviolet radiation. Once the exposed portions of the photoresist are hardened, a solvent is employed to develop the photoresist by washing away the unexposed portions covered by the mask during ultraviolet radiation. The photoresist thus covers the portions to be etched away in the final process. The exposed metal is then electroplated with the permanent mask. Once the permanent mask has been deposited, the hardened photoresist is washed away by hot organic strippers. A chemical etch is then used which does not etch the permanent mask but etches the now-exposed areas of the substrate previously covered by the hardened photoresist. In this process, electroplated nickel and beryllium-copper alloy is typically employed. The etchants used are ammonium hydroxide and cupric chloride or chromium trioxide and sulfuric acid. A difficulty with the bi-metal process is the expensive and limited types of materials which may be used for the process and to make up the substrate.